Do not delete the template when zeroing a dependant image.

Change-Id: I9e472334efa9dadb5389db70b0953f88b9be858a
Bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=910013
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12178
Tested-by: Haim Ateya <hateya@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>